Switching power converters using coupled inductors are known. For example, U.S. Pat. No. 6,362,986 to Schultz et al., which is incorporated herein by reference, discloses, among other things, multi-phase DC-to-DC converters including coupled inductors. These DC-to-DC converters typically have a higher effective switching frequency and lower switching ripple current magnitude than corresponding DC-to-DC converters using discrete (uncoupled) inductors.
FIG. 1 shows one prior art two-phase buck DC-to-DC converter 100 using a coupled inductor 102. Each phase 104 includes a switching device 106 electrically coupled between an input power source 108 and an inductor 110. A free wheeling device 112 is electrically coupled between inductor 110 and ground, in each phase 104. Each inductor 110 is electrically coupled to a common output node 114, which includes an output filter 116, such as a capacitor.
Each inductor 110 is part of common coupled inductor 102 shared among phases 104. In particular, each inductor 110 is magnetically coupled with each other inductor 110 via a magnetic core 118 of coupled inductor 102. Each inductor 110 has its own self inductance, often referred to as leakage inductance, which is critical to the operation of DC-to-DC converter 100. In particular, leakage inductance must be sufficiently large to prevent excessive switching ripple current magnitude. On the other hand, if leakage inductance is too large, DC-to-DC converter 100 will exhibit poor transient response. As taught in U.S. Pat. No. 6,362,986, magnetic coupling between inductors 110 in coupled inductor 102 should be sufficiently strong to realize the advantages associated with using a coupled inductor, instead of multiple discrete inductors, in DC-to-DC converter 100.
A controller 120 commands switching devices 106 to repeatedly switch between their conductive and non-conductive states to regulate the voltage magnitude on output node 114. Typically, controller 120 is configured so that switching devices 106 switch out of phase with respect to each other to promote cancellation of switching ripple current on output node 114. Free wheeling devices 112 provide a path for current through inductors 110 when switching devices 106 are in their non-conductive state.
The frequency at which controller 120 causes switching devices 106 to switch between their conductive and non-conductive states is referred to as the switching frequency of DC-to-DC converter 100. DC-to-DC converter 100 typically operates at a relatively high switching frequency, such as at least 100 kilohertz, to promote low ripple current magnitude, small size of coupled inductor 102, small size of output filter 116, and/or fast transient response of DC-to-DC converter 100. In particular, ripple current magnitude decreases as switching frequency increases, so ripple current magnitude can be decreased by increasing switching frequency. Low ripple current magnitude is typically desired because ripple current creates losses in components of DC-to-DC converter 100 and ripple voltage on output node 114.
Additionally, as discussed above, leakage inductance of inductors 110 must be sufficiently large so that ripple current magnitude is not excessively large. However, since ripple current magnitude decreases as switching frequency is increased, increasing switching frequency may allow leakage inductance of inductors 110 to be decreased while still maintaining an acceptable maximum ripple current magnitude. Decreasing leakage inductance advantageously improves DC-to-DC converter 100's transient response, and may allow coupled inductor 100 to be made smaller and/or cheaper. Furthermore, increasing switching frequency may allow size and/or cost of output filter 116 to be decreased.
Accordingly, there are significant advantages to operating a switching power converter using a coupled inductor at a high switching frequency. However, practical limitations typically prevent operating a switching power converter at as high of a switching frequency as desired. For example, core losses, which are losses in magnetic core 118 of coupled inductor 102 resulting from change in magnetic flux in core 118, typically increase with increasing switching frequency. Core losses are undesirable because they reduce efficiency of DC-to-DC converter 100 and may cause excessive heating of DC-to-DC converter 100. Thus, core losses in magnetic core 118 may prevent a switching power converter from being operated at as high of a switching frequency as desired.